1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, and is particularly suitably applicable to a single-lens reflex camera, a video camera, a television camera, a surveillance camera, and the like.
2. Description of the Related Art
Known focusing methods employed in zoom lenses include front-lens focusing in which a first lens unit provided nearest to an object-side end is moved, inner focusing or rear focusing in which a second or subsequent lens unit is moved, and so forth.
In a typical inner-focusing or rear-focusing zoom lens, the first lens unit has a smaller effective aperture than the first lens unit of a front-lens-focusing zoom lens and is therefore advantageous in the size reduction of the lens system as a whole. Furthermore, since focusing is performed by moving a relatively small and light lens unit, quick focusing is realized easily particularly in cameras having an autofocus function.
Known zoom lenses each having a generally small optical system and in which a high zoom ratio is easy to realize include a positive-lead zoom lens in which a lens unit having a positive refractive power is provided nearest to the object-side end.
For example, a known zoom lens includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power. This zoom lens performs zooming by changing air gaps between adjacent ones of the lens units.
Japanese Patent Laid-Open No. 59-52215 discloses a zoom lens that performs zooming by moving a second lens unit and a third lens unit and performs focusing by moving the third lens unit. Japanese Patent Laid-Open No. 2009-086537 discloses a zoom lens that performs zooming by moving a second lens unit and a third lens unit, performs focusing by moving the third lens unit, and performs image stabilization by moving a sub-lens unit included in a fourth lens unit in a direction perpendicular to the optical axis.
In an inner-focusing zoom lens, variations in aberrations during focusing are larger than in a front-lens-focusing zoom lens. Particularly, in focusing on a near object, various aberrations such as spherical aberration tend to become large.
In an inner-focusing positive-lead zoom lens, it is important to reduce variations in aberrations during focusing and to provide good optical performance for all objects that are at any distances while realizing a high zoom ratio and a size reduction of the lens system as a whole. To achieve this, it is important to appropriately set the refractive powers and the lens configurations of the lens units and the conditions under which the lens units are moved during zooming.
Particularly, it is important to appropriately set the lens configurations and so forth of a lens unit that is used for focusing and lens units that are provided on the image side of the lens unit used for focusing. Unless the configurations of these lens units are set appropriately, it is very difficult to provide a zoom lens having high optical performance at all zooming positions and at all object distances while realizing a high zoom ratio.